The present invention relates to a foamable silicone rubber composition and a method for curing the composition to give a cured and foamed cellular silicone rubber article. More particularly, the invention relates to a silicone rubber composition which can be foamed and cured into a cellular silicone rubber article without using any external heat source as well as to a novel and efficient method for simultaneously foaming and curing such a foamable silicone rubber composition in a continuous process as in the vulcanization following extrusion molding of the composition.
Several different processes are practiced in the prior art when a silicone rubber composition is desired to be cured or vulcanized in a continuous process to give a continuous-length cured silicone rubber body such as tubes including the hot-air vulcanization (HAV) method under normal pressure, continuous steam vulcanization (CV) method, liquid curing medium (LCM) method and the like. When a continuous-length cured silicone rubber body having a foamed and cellular structure is desired, however, these conventional methods are not always quite satisfactory in respect of the controllability of the process and obtaining a fine and uniform structure of the cured body.
Apart from silicone rubber compositions, there is an increasing general demand in recent years for various kinds of continuous-length thick-walled foamed rubber bodies in applications for gaskets and heat insulators in building works, rollers of a foamed rubber used, for example, as a fixing roller in xerographic copying machines and the like and a method for manufacturing such foamed rubber bodies with stability and low cost is eagerly desired to follow extrusion molding of the rubber composition. For example, a method is proposed and practiced in which a rubber composition of an ethylene-propylene-diene terpolymeric rubber, i.e. a so-called EPDM rubber, or a polychloroprene rubber is continuously extruded out of an extruder machine and then irradiated with ultrahigh-frequency electromagnetic waves so that the rubber composition absorbs the energy of the electromagnetic waves and heated up to the vulcanization temperature to be cured with simultaneous foaming by the decomposition of the blowing agent contained in the rubber composition. This method of irradiation with ultrahigh-frequency electromagnetic waves, referred to as the UHFV method hereinbelow, is considered not to be applicable to the continuous curing of silicone rubber compositions becuase the loss index in silicone rubbers is generally small not to ensure sufficient energy absorption of the electromagnetic waves.
The above mentioned UHFV method is performed usually at a frequency of 2450.+-.50 MHz or 915.+-.25 MHz so that it is essential that the rubber composition can absorb the energy of the electromagnetic waves of these frequencies in a high efficiency to be rapidly heated up to the vulcanization temperature. The energy P absorbed by a dielectric material under irradiation of UHF waves ogenerated in a microwave generator is given by the equation: EQU P=(5/9)f.multidot.E.sup.2 .multidot..epsilon..multidot.tan.delta..times.10.sup.10,
in which P is the energy absorbed and converted into heat in watts/m.sup.3 ; f is the frequency in Hz; E is the high-frequency electric field in volts/m; .epsilon. is the dielectric constant; and tan .delta. is the dielectric loss factor. The product of (.epsilon..multidot.tan.delta.) is called the loss index of the material. It is known that, in order that the UHFV method can be successfully applied to the foaming vulcanization of a rubber composition, the loss index of the rubber composition should desirably be at least 0.08 or, if possible, at least 0.2. To the contrary to this requirement, silicone rubber compositions generally have a very small loss index of only about 0.03 at a frequency of 3000 MHz. This is the reason for the general understanding that the UHFV method is not applicable to the foaming vulcanization of silicone rubber compositions.
Several attempts and proposals have been made in the prior art to obtain a silicone rubber composition having an increased loss index to meet the above mentioned requirement for the UHFV method. These prior art proposals are directed mainly to the modification of the organopolysiloxane as the principal ingredient of the silicone rubber composition. For example, Japanese Patent Kokai No. 52-37966 discloses an organopolysiloxane of which at least 5% by moles of the organic groups bonded to the silicon atoms are aliphatic hydrocarbon groups with substitution of aryl groups, chlorine atoms, fluorine atoms, mercapto groups or methylol groups or alkoxy groups. Such a modified organopolysiloxane, however, is not practical in respect of the great decrease in the heat resistance, weatherability, electric properties and surface properties of the cured silicone rubber which are the characteristics inherent in silicone rubbers in general.